Mark Node::opcode() and Operator::opcode() as constexpr.

Without the explicit constexpr keyword, Clang seems to be able to treat
these methods as constexpr, whereas MSVC will not.

Bug: v8:11760
Change-Id: I9f6492f38fb50dcaf7a4f09da0bd79c0da6a50eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2912916
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74791}

src/compiler/node.h

@@ -50,7 +50,7 @@ class V8_EXPORT_PRIVATE Node final {
 
   const Operator* op() const { return op_; }
 
-  IrOpcode::Value opcode() const {
+  constexpr IrOpcode::Value opcode() const {
     DCHECK_GE(IrOpcode::kLast, op_->opcode());
     return static_cast<IrOpcode::Value>(op_->opcode());
   }

src/compiler/operator.h

@@ -73,7 +73,7 @@ class V8_EXPORT_PRIVATE Operator : public NON_EXPORTED_BASE(ZoneObject) {
   // A small integer unique to all instances of a particular kind of operator,
   // useful for quick matching for specific kinds of operators. For fast access
   // the opcode is stored directly in the operator object.
-  Opcode opcode() const { return opcode_; }
+  constexpr Opcode opcode() const { return opcode_; }
 
   // Returns a constant string representing the mnemonic of the operator,
   // without the static parameters. Useful for debugging.